Abstract - Small Animal Models Core The objective of the Small Animals Model Core is to generate a series of mouse models that express the unmutated common ancestor (UCA) or intermediate antibodies (IAs) of the DH511 lineage of broadly neutralizing antibody (bnAb). These mouse models will be used to test the efficacy of immunogens that are designed to elicit the DH511 lineage of bnAbs. Toward this end, the Animal Model Core will generate two types of mouse models. The first type of model expresses pre-rearranged V(D)J exons of the DH511 antibodies. Owing to allelic exclusion, the pre-rearranged DH511 V(D)J exon will inhibit the rearrangement of endogenous mouse immunoglobulin (Ig) loci. As a result, B cells in these mice will express predominantly DH511 antibodies. One common problem for this type of mouse model is that tolerance control mechanisms delete B cells expressing the knock-in human Ig genes. To overcome this hurdle, we have developed a method to express human antibodies conditionally in mature B cells, thereby circumventing tolerance control during B cell maturation. If necessary, we will employ this conditional expression system to generate DH511 mouse models. Since this type of model provides a large population of B cells expressing DH511UCA or IAs, it would serve as a sensitive assay for the initial evaluation of immunogens. However, the system does not recapitulate the complexity of physiological B cell repertoire in at least two major respects. First, the unique UCA expressed in the mouse model may not be present in a fraction of human populations. Second, the system lacks competing antibodies for irrelevant epitopes. To address these limitations, the Animal Model Core will generate a second type of mouse model where the VH3-15, D3-3 and JH6 gene segments of DH511 undergo de novo V(D)J recombination. Due to junctional diversity, the recombination will create a wide range of CDR H3s, some of which may be suitable for the development of DH511-like antibodies. Moreover, the system does not preclude the rearrangements of endogenous mouse Ig gene segments. Therefore, the B cell repertoire of this mouse model will consist of potential DH511 precursors as well as other human VH3-15/D3-3/JH6 and mouse antibodies. Immunization of this mouse model could assess the ability of the immunogen to select for and mature DH511 precursors in the context of complex antibody repertoires.